How to make an oscilliscope out of an old monitor

Submitted by Mark on March 16, 2007 - 3:26pm.


So I'm sitting there looking up at the random stacks of computer hardware I have in my loft. A couple of NeXTstations, a few Apple IIs, half a dozen Macs from the 90s....and as I'm looking I come upon this old IBM monitor that I saved from destruction several years ago.

So I got to thinking that I would turn this into an oscilloscope.

Here's the monitor, an IBM PCJr color monitor from 1985.


To start, off, here is basically how a CRT display basically works:



A gun at the back of the screen shoots out a beam of electrons towards a fluorescent screen, and two electromagnetic deflector coils move the beam up and down 60 times a second, and side to side 31500 times a second, to form an image on the screen.

So here is the back all opened up on our IBM monitor.


Deflector Coil Assembly


Basically, there are four wires leading up to two deflector coils in the neck of the CRT. In this case, red and blue control the horizontal sync while yellow and brown control the vertical sync. You can find out which is which by disconnecting them and reconnecting them, and seeing what happens to the picture. If you get a vertical line, you probably just cut one of the horizontal signal wires, and vice-versa.

Oscilloscope wiring diagram, vertical configuration


After some trial and error, I figured that by keeping the vertical sync connected, and replacing the horizontal sync with an audio signal from an amplifier, you get a nice little oscilloscope:


Vertical Configuration (1 height = 60Hz)


I tried it reverse too; replacing the vertical sync with the audio input and keeping the horizontal sync connected to the monitor supply, but this didn't work as well. I ended up with a horizontal line that would just move up and down with the amplitude of the music. It didn't really show any waves.

 
Horizontal Configuration (1 width = around 30KHz)


This is because the original vertical sync signal was 60Hz, while the original horizontal sync signal was something like 31500Hz (60Hz times 525 lines). So the vertical configuration would show a 60Hz wave from peak to valley (picture below), and other waves in the audible spectrum, with 120Hz being half of that, 240Hz being half of that and so on.


60Hz sine wave on the vertical oscilloscope
I also looked up the pinout of the PCJr proprietary monitor connector so I could change the colors. You short the intensity pin to the color that you want. The below example gives me red.


PCjr display connector

At some point, I might make a circuit that changes the colors with the amplitude of the music. Which would be pretty neat.

Here are some shots of the oscilloscope that I took in a dark room:

60Hz sine wave 120Hz sine wave 240Hz sine wave

And there you have it. It's not a replacement for an actual calibrated scope, but it certainly looks cool. Here's a video of the oscilloscope in action:

 

 And below is a later video I made of it playing Bohemian Rhapsody by Queen

 

»
Tags:

okay, so how are you doing

Submitted by Maria on March 16, 2007 - 9:14pm.
okay, so how are you doing this, out of curiosity?

okay never mind, i was being

Submitted by Anonymous on March 16, 2007 - 9:14pm.
okay never mind, i was being too lazy to scroll down

you should make it change colors too

Submitted by linuxpenguin on March 27, 2007 - 5:15pm.
i was about to say that you should see if you can make it change colors with the music too, that would be pretty cool, but then i noticed you said you were thinking of doing that. . .

which would be pretty cool!

i think it would be funny to

Submitted by linuxpenguin on March 27, 2007 - 5:33pm.
i think it would be funny to hook it up to a computer with text to speech software and make it talk to people like it was some computer from an old scifi movie or something.

(of course you'd have to use VNC to control the computer, not the monitor :) )

Course, you could just ssh

Submitted by clarjon1 on March 4, 2008 - 10:27am.

Course, you could just ssh into the box if it's a linux box, and
echo "hello, dave" | festival --tts

oscilloscope demo

Submitted by Tim McCaskey on April 27, 2007 - 1:11pm.

Mark, I enjoyed seeing the oscilloscope in action today. It was good work, and it seemed you had fun doing it. Good luck with the particle accelerator...

vga

Submitted by mark W on August 6, 2007 - 1:45pm.

now, that is cool

one thing that I am thinking of is something like a little box that you could connect to a vga monitor. the box could have an oscillator to connect to the v-sync pin, and an audio input to connect to the h-sync. you could even have 3 potentiometers connecting the r,g, and b pins to .7v (or 1v, i cant remember), to get various colors.

but would this actually work? I dont think so, because a vga monitor's built in smarts would probably freak it out, what with the constantly changing h-sync. but it would be cool.

question: do I have to use an old-fashioned cga monitor in order to do this? or can I use a vga monitor, and hack it apart similar to what you have done here?

I dont have anything except for more modern vga monitors, but would really like to try this out!

To work on a VGA monitor,

Submitted by Mark on August 6, 2007 - 2:49pm.

To work on a VGA monitor, you would have to provide a dummy signal to get something on the screen, because without a computer, it won't put anything up there initially.

The CGA monitor was easy because when you fired it up, it had a blank, white screen by default.

You should: 1. completely

Submitted by Anonymous on January 13, 2008 - 11:45am.

You should:
1. completely remove the board under the crt.
2: Unsolder the flyback and connect it to a transistor to make that it generates the EAT(20-40kv) and 210 v for the anodes.Power it up with 12V
3: Connect the transformer pins in the corrects pins of the crt(DON'T USE A TESTER IT WILL BLOW UP WITH 20KV or WITH 200V high freq.)
4:Connect the h sync to a variable sawtooth oscillator(timebase)
5:Connect the vsync to a variable gain bf amplifier

cga monitor...

Submitted by mark w on August 6, 2007 - 4:32pm.

well, I'm sure that I could rouse one up from somewhere.

Although, in all honesty, I'm fairly sure that I've never seen a cga monitor before now (I may have, although I wouldn't have recognized it).

i just made one with a

Submitted by Anonymous on August 18, 2008 - 3:36am.

i just made one with a little portable tv and a guitar battery powered amp it works great!!!!! its a jwin tv and a epiphone ep-1 amp an the amp has the speaker cabinet detached, so its just the part with the nobs, and it is taped to the top of the tv.